Handle unlocked overriden inputs

This fixes the error

  in pure evaluation mode, 'fetchTree' requires a locked input

when using '--override-input X Y' where Y is an unlocked input (e.g. a
dirty Git tree).

Also, make LockFile use ref instead of std::shared_ptr.

tests/flakes/unlocked-override.sh

@@ -0,0 +1,30 @@
+source ./common.sh
+
+requireGit
+
+flake1Dir=$TEST_ROOT/flake1
+flake2Dir=$TEST_ROOT/flake2
+
+createGitRepo $flake1Dir
+cat > $flake1Dir/flake.nix <<EOF
+{
+    outputs = { self }: { x = import ./x.nix; };
+}
+EOF
+echo 123 > $flake1Dir/x.nix
+git -C $flake1Dir add flake.nix x.nix
+git -C $flake1Dir commit -m Initial
+
+createGitRepo $flake2Dir
+cat > $flake2Dir/flake.nix <<EOF
+{
+    outputs = { self, flake1 }: { x = flake1.x; };
+}
+EOF
+git -C $flake2Dir add flake.nix
+
+[[ $(nix eval --json $flake2Dir#x --override-input flake1 $TEST_ROOT/flake1) = 123 ]]
+
+echo 456 > $flake1Dir/x.nix
+
+[[ $(nix eval --json $flake2Dir#x --override-input flake1 $TEST_ROOT/flake1) = 456 ]]

tests/local.mk

@@ -7,6 +7,7 @@ nix_tests = \
   flakes/follow-paths.sh \
   flakes/bundle.sh \
   flakes/check.sh \
+  flakes/unlocked-override.sh \
   ca/gc.sh \
   gc.sh \
   remote-store.sh \